It's clearly Sharon, from the fact that everybody other than Brie is British (and Brie just did a role in The Five-Year Engagement) with what was supposedly a pretty good accent).

What's funny about Brie is that when the Russos took charge I thought that Gillian Jacobs is actually a pretty good match for the comics version of Sharon, but since the movie Carters are brunettes, that's not the case here.